when did this condition start to happen? when you pull the spark plug after an attempt to start it, does the elctrodes look wetted over?
ok, most likely there is some buildup in the pilot jet. this is a jet that is in the float chamber of the carb it is responsible for idle speeds and aids in starting as well. this jet plugs very easily. and is known to start to do this as little as 3 weeks or so depending on fuel quality. but first, just to make sure you are getting fuel to the carb, if you look at the bottom of the carb there is a drain screw for the float bowl. go ahead and unscrew it and see that fuel flows out. if none does, then you may have a blockage in the fuel line, fuel tank, or the float valve in the carb may have stuck closed. again that is a condition from buildup in the carb from sitting. but most likely you are dealing with a plugged jet. if you do get fuel flow, then the problem is in the carb. also if you notice where the choke plunger goes into the carb, shoot carb cleaner in the hole the choke plunger needle seats into. ensure that carb cleaner comes back out the tube into the float chamber as well. this can clog just as easily, and cause a no start.
you will need to remove the carb from the atv, and remove the bottom float bowl. at that point you will see tubes that come down into that float area. those have fuel jets in them. the smallest orifice one is the pilot jet. that is one that you will need to concentrate on. take a small gauge wire. something compared to a wire brush bristle, and clean through that jet as well as with some aresol carb cleaner. oh and you may want to removed it from the carb to do this so dirt doesnt get internally intot he carb. but clean that jet out very well. the smallest amount of buildup makes a big difference. once that is clean shoot some carb cleaner in the hole that the jet goes into. and see that carb cleaner exits out into the throat of the carb through a small hole. just to make sure the way is clear.
doing these things will get your atv back to starting on its own again. it will also help if you clean the main jet(the larger jet) as well just incase this has started to get some buildup as well.
hope this helps. any questions about this please ask, ill be happy to help. thank you.
the symptom we have to go off of, is that plug is not showing any fuel. really that can happen for a couple reasons. but the most common, and especially after a storage period would be the carb. if you are certain that this carb is clean and setup properly, the air cleaner is in good shape, airbox is fully intact. then its possible that there could be some issues with either the valveing, or low compression or something like that. but those are absolute last case scenerios. the carb and intake is really that only that at this point that can be done externally to help cure this. if it cannot be cure with then maybe a top end inspection could be warranted. but i would exhaust all efforts with the intake. if you run into an issue where you are not getting spark, then that can put this into a different direction. but the cylinder is not getting fuel in it, whether it is the carb not supplying the fuel or the intake not pulling fuel into it.
the pilot jet in the carb is what will cure that. but as far as tuning. if you get a popping noise when it comes back down off throttle or it doesnt want to go back down to an idle, or surgeing it is too lean. if it seems to load up after prolong idleing it is too rich. with the pilot jet cleaned. that fuel mixture screw that is at the front part of the carb, that is pointing downward, that can be set at 2.5 turns out from seated. this is a good general setting, and should produce an idle speed, if not then take a good look at that pilot jet.